Override QEMUVGA for Atomic nightly compose tests
Sigh. RHBZ#1518464 isn't fixed for the Atomic nightly composes
we test, so we can't switch back to 'std' for these, yet at
least. Let's override the video driver to 'virtio' for these
(instead of qxl, as we've hopefully got variant needles for
all the #1403365 cases now).
Signed-off-by: Adam Williamson <email@example.com>